Screening

Screening

Programme to detect unsuspected disease in a population of apparently healthy people

Surveillance

Programme to detect disease in a population already with disease

 

Important considerations

  1. Disease
    • Common
    • Important
    • Long premobid latent period
    • Detectable at early stage
    • Treatable: by defined principles, cost-effective
  2. Test
    • Sensitive (ability to detect)
    • Specific (ability to exclude others)
    • Non-invasive
    • Acceptable to patients
    • Cost-effective
    • Does no significant harm to patients

Examples

  1. Breast cancer
    • All women over 50-64 advised to have mammogram every 3 years.
    • Mammographic abnormalities referred to breast specialist for clinical examination + further investigations
    • 5-10% breast cancer familial
    • Genetics: BrCA1 (chromosome 17), BrCA(chromosome 13), p53(chromosome 17), Ataxia telangectasia gene
    • Detected cancers: smaller, CIS, well differentiated (ie. all rather good prognostic factors)
  2. Ovarian cancer
    • Two or more 1st degree relatives
    • BrCA1,BrCA2 genes
  3. Cervical cancer
    • 3 yearly Papanicolau smears
    • CIN 1,2,3
    • Treated with cone excision biopsy
  4. Colorectal cancer: at risk families, polyps, IBD
    • Faecal occult blood-testing kit, plus repeat test
    • If positive > colonoscopy or double contrast barium enema
  5. Abdominal aortic aneurysms
  6. Congenital dislocation of hip
    • Ortolani
    • Barlow's
  7. Prenatal screening

Bias in screening

  1. Lead-time bias: Survival measured from detection to death will be longer (cause it's detected earlier)
  2. Selection bias: Individuals who take up screening are more health conscious
  3. Length bias: slowly growing tumours more likely to be detected by screening than rapidly growing tumours between screening intervals

Problems in screening

  • Increased morbidity with unaffected prognosis
  • Excessive therapy of doubtful cases
  • Increased anxiety
  • Lack of target population co-operation
  • Costs
  • Inffective screening tests
  • False reassurance